The present invention generally relates to techniques for rf pulse compression and more particularly to high power pulse compressors using cavity resonators to store and release pulse power in the aid of pulse compression.
High Q cavity resonators are used in pulse compressors to store a significant part of the energy from an amplified pulse received from an rf pulse power source. During a “fill time,” the pulse power incident on an aperture in the cavity resonator has part of this power reflected and the remaining part coupled to the cavity. Some of the power coupled to the cavity is dissipated in the cavity walls, while the balance builds energy in the cavity resonator. By abruptly reversing the phase of the rf power source, the energy stored in the resonator cavity is released and effectively combined with the power from the rf source to yield an increase in peak power output at a reduced pulse length.
The high Q cavity resonators such as used in high power pulse compressor systems have typically been cylindrical cavities for supporting modes of the TE01n family. These modes have electric fields that do not terminate on the cavity walls. Hence, electron emission and multipactor effects are significantly reduced as compared to many other modes.
An example of the use of high power rf pulse compressors is the SLED (Stanford Linear Energy Doubler) system used by the Stanford Linear Accelerator (SLAC) to increase the energy in the beam used for particle acceleration. SLED uses pulse compressors comprised of two cylindrical cavity resonators operating in a single mode, and a four port 3 db sidewall hybrid having ports conventionally denoted ports 1, 2, 3, 4. In this type of system, pulse power fed into port 1 of the sidewall hybrid is divided equally into ports 2 and 3 with a phase difference of 90° between Ports 2 and 3. The output from port 2 couples to one of the cylindrical cavity resonators, while the output from port 3 is coupled to the other of the cylindrical cavity resonators. The transmission line lengths used to feed the two resonator cavities of the SLED system are equivalent to maintain the 90° phase differential at and within the cavities.
In the SLED system, each cavity reflects essentially all of the incident power at the start of the filling pulse. The reflection travels back to the hybrid and combines to exit the hybrid at the hybrid's port number 4. As the cavity starts to fill, less power is reflected and more power is coupled. All this is a function of time, frequency, cavity Q, and coupling coefficient in a predictable manner. After an appropriate fill time interval, the phase of the rf power to hybrid port 1 is reversed. (This is usually accomplished by reversing the phase of the drive to the rf power source, such as a klystron amplifier.) Immediately after phase reversal, the following occurs at the cavities: (1) the incident power is completely reflected, and (2) power is emitted from the cavity's stored energy. The phase of the reflected power and the emitted power are equal at a given cavity, so that the voltages add. Consequently, the power traveling from a cavity to the hybrid increases as the square of this voltage. The hybrid serves to combine the reflected power and power released from energy stored in the cavities, and to deliver this power to port 4 of the hybrid.
In another type of pulse compression system a single resonator cavity is fed by a 3 port circulator instead of a 4 port hybrid. In this case, power from the rf source is fed into port 1 of the circulator and emerges out of port 2. Port 2 is connected to a transmission line that is, in turn, coupled to the cavity resonator to build energy in the single cavity during the pulse fill time. Upon reversal of polarity of the input pulse power at port 1 of the circulator, the stored energy released from the single cavity resonator is released to aid in the production of a compressed output pulse, which is conveyed out of port 3 of the circulator to a load.
The advantage of a single cavity/circulator system is that it eliminates the need for the two cavity resonators used in the SLED system. A disadvantage is that circulators are more complex and costly as compared to four port hybrids. Also, because all of the energy is stored in a single mode in one cavity, the peak electric field within the cavity of the circulator fed single cavity system is increased by the square root of 2 over the two cavity/hybrid pulse compressor. The power level in the feed line connected to port 2 of the circulator is also increased.
The present invention provides the benefits of both of the above described pulse compression systems into a single system. That is, the invention involves a single cavity resonator that is hybrid fed. Thus, the maximum electric fields in the cavity resonator and the feed arms to the resonator are equivalent to the two cavity/hybrid system. A hybrid can be used instead of the more costly circulator, and a single cavity resonator is required instead of two.